I think it depends on what the dog thinks they're doing. If you just stick your penis in their mouth they will be unlikely to bite it because they're not doing anything right then. If the dog is licking you and you push into their mouth, it is more likely they would nip you because they're using their mouth for the licking motion and not expecting it.
You're probably fine and you can test the waters with your finger first. Most dogs would just go "bleh" I didn't ask for this and spit you out.